Wednesday, August 31, 2016

SSPA Power Supply - New Thermistor Success!

The new 10 Ohm 18 amp Thermistors performed exactly as I had hoped.  Actually they really exceeded my expectations.  With no step/soft-start 4 of the dps-1200fb's starting up at the same time consume nearly 11 amps of inrush @ 240 vac.  My Supply has 8 of them so that would be abut 22 amps of inrush.  I'm not really comfortable with that much, it's not terrible, but I just didn't like it.

So...I setup a step start that uses Thermistors and a relay to short around them after 3 seconds.  3 seconds is way longer than what's needed, but it just happens that the timer delay's I have start off at 3 seconds, so I'm using them.  It's not hurting anything to let the Thermistors be inline that long.  A lot of designers don't even bother to short around them with a relay.  Thermistors get pretty warm under a load, and if they're hot when the system is turned off, they won't have enough resistance to perform a soft start.  As they got hot they gradually loose almost all their resistance.  

The relays I'm using short around them after 3 seconds allowing them to remain and stay cool so that if them amp has to be turned off and then on again immediately, the soft start will work every time. the details...

I replaced the 1 Ohm Thermistors which were getting inrush around 8.5 amps.  I replaced them with 10 Ohm today and the results are really pleasing.  1.4 amps!

And that's repeatable over and over.